Posted
by
Soulskill
on Friday November 30, 2012 @03:30PM
from the fat-guy-little-coat dept.

An anonymous reader sent word that astronomers have discovered an absolutely enormous black hole residing in a galaxy that seems too small for it. In a new study (PDF), researchers looked at galaxy NGC 1277 and found that its central black hole weighed in at roughly 17 billion solar masses. Quoting Phil Plait: "The problem is, that’s far more massive than the central bulge of NGC 1277 would suggest the black hole should be. It’s well over half the total mass of the bulge! In fact, the entire mass of the galaxy is about 120 billion solar masses, which means the black hole at its heart is 14 percent of the total galaxy’s mass; compare that to the Milky Way’s black hole mass of 0.01 percent and you’ll see why astronomers were shocked."

I would think it more likely that a supermassive blackhole was ejected by a galactic collision and landed in a dwarf galaxy where its living out its retirement. Hhhmmm, sounds vaguely like the Mel Gibson story...

It's not quite that simple. The usual relativistic treatment of black holes seems to use a point source simplification (like you do with Newtonian gravity in high school). All the mass is assumed to be concentrated in the singularity, at the centre of the black hole. If that's the case then you're correct - the black hole consists of empty space with a point of infinite density at the middle.

But how exactly do you achieve that? The event horizon, among other things, is where time stops from the perspect

Well yes if you use the size of the event horizon and the mass of the black hole to calculate density then you get a low density.

But the mass is not distributed over that volume. inside the black hole the mass is actually contained in an infintesimal point, and the density is infinite. At least according to the math; it's impossible to look inside the event horizon to find out if that's really the case.

At the very least it's clear that a black hole must have density significantly higher than that of a neutron star. Saying it's less dense than the air is misleading in that respect.

But the mass is not distributed over that volume. inside the black hole the mass is actually contained in an infintesimal point, and the density is infinite. At least according to the math;

Ah yes, but it also can't be, since time travels slower and slower the deeper you go, it would take infinite time for matter to actually reach that point. The actual matter distribution depends on a lot of things, mostly how it was formed. Presumably, there is at least a central region with density greater than a neutron star, but if the majority of the matter in the star was added later, or was otherwise distributed throughout the whole region of the black hole during or after it's formation, that matter w

"But the mass is not distributed over that volume. inside the black hole the mass is actually contained in an infintesimal point"

Ahh! the old problem... equations versus reality!

All that the Einstenian equations tell us is that they don't know how to manage black holes beyond the event horizon (and that they are wrong about them because of that).

Given that the event horizon neatly divides the universe in two, it is perfectly reasonable to say that the black hole density (from the outer univese perspetive) averages its overall percieved mass by its volume.

At the very least it's clear that a black hole must have "density significantly higher than that of a neutron star."

Because?

All you can say is that *if* (a big if) black holes behave more or less like all the physics we know about, there must be something within the black hole with densities above those we can find on a neutron star because by all we "common sense" know, black holes are like neutron stars, only more so.

"Saying it's less dense than the air is misleading in that respect."

What's misleading about saying density is defined as mass against volume?

All that the Einstenian equations tell us is that they don't know how to manage black holes beyond the event horizon (and that they are wrong about them because of that).

Ahh, the old problem of equations versus your imagination of what reality might be.;)

Einstein's equations work just fine inside the event horizon. It's the actual singularity itself which raises some eyebrows. And even then, we don't actually know that such a thing isn't possible in reality. But you are uncomfortable with the idea, therefore they're wrong. Got it.

At the very least it's clear that a black hole must have "density significantly higher than that of a neutron star."

Because?

Because it's required by those pesky equations.

All you can say is that *if* (a big if) black holes behave more or less like all the physics we know about

Everything said about just about anything can be presumed to have an "as long as our understan

This is just an example of a MaCHO [wikipedia.org]. We've theorized about them for a while. They are a strong candidate for a bulk of the dark matter we've detected. The other candidates are WIMPs [wikipedia.org].

This is just an example of a MaCHO [wikipedia.org]. We've theorized about them for a while. They are a strong candidate for a bulk of the dark matter we've detected. The other candidates are WIMPs [wikipedia.org].

Uh, no. MaCHOs were supposed to be Jupiter-size to brown dwarf-size lumps of mass, careening through galaxies without being associated with stars or other luminous matter. A black hole *can* count as a MaCHO *if* it has no accretion disk, but we think most black holes have accretion disks and therefore emit X-rays (and thus don't count as dark matter). This black hole is firmly in the not-a-MaCHO category; for that matter, what we today know about Big Bang baryogenesis pretty strongly rules out MaCHOs be

There are actually two kinds of missing matter at this point. Evidence based on things from the Big Bang like the cosmic microwave background and relative abundance of light elements gives us an idea what portion of the mass in the universe would be made up from baryonic matter (things like protons and neutrons, so pretty much anything made of atoms), and then there is a large portion of mass that needs to be made up of something else, that is what gets called dark matter. Of the portion we think that is

I read (I think it was in 'death by black hole') that the more massive the black hole, the less gravity you experience at the event horizon. For a 1 trillion mass black hole, supposedly it would only have 10g at its event horizon. For still greater masses, you could have 1g, something reasonable for both a human and a spaceship to deal with... in theory, you could hover a ship with a person in it at the very boundary of such an event horizon... how sharp would this boundary be? I'd lower a string to see where and how it gets clipped.